The long-term goal of this project is to understand the cellular and molecular mechanisms that underlie the formation of arteriovenous malformations (AVMs) in order to identify novel candidates for therapeutic targets. AVMs are severe vascular anomalies characterized by the formation of direct shunts from arteries to veins that displace normal capillary beds. AVMs are prone to rupture and dangerous when they occur in crucial organs such as the brain, and are an important source of neurological morbidity in relatively young people. Current treatment options are limited, and carry risks of their own. Although most AVMs are sporadic, mutations in the HHT (Hereditary Hemorrhagic Telangiectasia) genes are one of the few genetic causes of AVMs which have been identified, and can lead to an autosomal dominant disorder characterized by the formation of AVMs. HHT1 (endoglin) and HHT2 (Alk1, activin receptor-like kinase) are receptors for ligands of the TGF pathway superfamily and are expressed primarily on endothelial cells (ECs) in the vasculature. Mutations in the Notch pathway also lead to the formation of AVMs in animal models. These findings combined with data showing interactions between the TGF and Notch pathways raise the question of whether Notch and Alk1 may interact to mediate AVM formation. As the mechanisms of AVM pathogenesis are complicated and still not well characterized, we propose to use the embryonic dorsal aorta (DA) and cardinal vein (CV), which are the first artery vein pair to form in the body, as a model system to investigate Notch and Alk1 interactions. In Aim 1, we hypothesize that Notch and Alk1 function in the same genetic pathway to mediate DA and CV development. We will test this hypothesis by 1) comparing the effects of a loss-of-function (LOF) mutation in Notch to that of LOF mutations in Alk1, and 2) by examining the consequences of a double mutant in both Alk1 and Notch. In Aim 2, we hypothesize that Notch is functionally downstream of Alk1 in formation of the DA and the CV. We will examine our hypothesis by testing whether Notch gene activity can override the effect of a LOF mutation in Alk1. In Aim 3, we will determine whether Alk1 activates the Notch pathway in ECs in a cell autonomous manner. We will examine whether Notch signaling is dependent on Alk1 expression at the level of individual cells. We believe that understanding the function of the HHT genes during normal development will lead to a better understanding of the pathways that are disrupted during the pathogenesis of AVMs.